1. Field of the Invention
The present invention relates to a swirl forming device in a combustion engine, which forms a swirling flow of the charge mixture (i.e., the air-fuel mixture) within the combustion chamber when the combustion engine is operated at a low load condition such as occurring during idling and abrupt deceleration.
2. Description of the Prior Art
An combustion engine which uses a lean mixture of fuel and air has long been known in the art. In this combustion engine operating with the lean air-fuel mixture, during a low load operating condition of the engine the lean air-fuel mixture is supplied and, at the same time, the swirl of the lean air-fuel mixture is developed within the combustion chamber, for the purpose of increasing combustion efficiency and, also, reducing fuel consumption.
In recent years, a swirl forming device has been suggested for an combustion engine of a double intake valve type, in which two bypass passages, through which air from the atmosphere bypasses the throttle valve, are employed. Those bypass passages have exit openings communicated with respective portions of the intake passages adjacent the intake ports that are positioned downstream of the throttle valve with respect to the direction of flow of an intake gas of the engine, so that the air flowing through those bypass passages can be jetted into the combustion chamber to form a swirl of the intake gas. See, for example, the Japanese Laid-open Patent Publication No. 2003-113719.
In the conventional swirl forming device of the type referred to above, the air flowing through the bypass passages is merged at a portion of the intake passage upstream of the intake valves with the air-fuel mixture then flowing through the main intake passage and is then introduced into the combustion chamber from a portion of each port adjacent the outer periphery of the combustion chamber. Accordingly, it does not appear that the position, at which the air flowing through the bypass passages is introduced into the combustion chamber, is well defined to achieve an efficient development of the swirl. In other words, the air flowing through the bypass passages and then jetted into the combustion chamber from a position adjacent the outer periphery of the combustion chamber collides against the peripheral surface of the combustion chamber and the top face of a reciprocating piston and, therefore, no swirling flow of the charge mixture can be smoothly developed.
Thus, the conventional swirl forming device of the design discussed above acts to develop the swirl of the intake gas merely by biasing the flow of the intake gas (the air-fuel mixture and the air) being introduced into the combustion chamber through the intake valves and, therefore, it has been found difficult to promote a vigorous and massive swirling motion. For this reason, with the conventional swirl forming device, the combustion efficiency cannot be sufficiently increased particularly when the lean air-fuel mixture is employed, and, therefore, hydrocarbons in the exhaust gas cannot be sufficiently reduced.